Since polypropylene resins are light in weight and excellent in recycling efficiency, there is an increasing demand for them as automobile parts. They are, for example, polypropylene resin compositions obtained by adding, to a crystalline polypropylene resin, an ethylene thermoplastic elastomer component such as ethylene•propylene copolymer or ethylene•butene copolymer and an inorganic filler such as talc. It is proposed to improve the moldability, mechanical physical properties, appearance and the like of such propylene resin compositions by using a polypropylene resin, elastomer component and inorganic filler selected as needed depending on the using purpose.
In particular, it has been elucidated that so-called “polymer blend” is effective for heightening the dispersibility of the ethylene thermoplastic elastomer component (especially, ethylene•propylene copolymer) to the crystalline propylene resin. It is well known to those skilled in the art that use of a propylene•ethylene block copolymer (which may hereinafter be called “ICP”) available by preparing a crystalline propylene resin (which may hereinafter be called “crystal component”) in the first step of polymerization, followed by the preparation of an ethylene•propylene random copolymer (which may hereinafter be called “rubber component”) in the second step of polymerization is effective.
Recently, there is a demand for the development of polypropylene resin compositions which can be molded into a thinner molding in a shorter molding time. One of the means for satisfying this demand is a technology using so-called “high fluidity material”, that is, ICP having an increased melt flow rate (MFR). Such a high fluidity material has improved moldability or formability and from this material, thinner moldings can be obtained, but flow marks tend to appear. The term “flow marks” means tiger stripes appearing on the surface of the moldings. Moldings having flow marks on their surfaces are not suited for designing and they therefore have a markedly reduced commercial value.
Flow mark lessening technology is disclosed, for example, in Patent Document 1, Patent Document 2 or Patent Document 3.
An object of Patent Document 1 is to provide a propylene resin composition well balanced in moldability or formability and physical properties such as rigidity and impact strength. In the means for resolution, ICP (Component a) having, in its entirety, an MFR of from 10 to 130 g/10 min and having a rubber component with a weight average molecular weight of from 200000 to 1000000 is disclosed. Also another ICP (Component b) having, in its entirety, an MFR of from 0.1 to 8 g/10 min and having a rubber component with a weight average molecular weight of from 300000 to 900000 is disclosed. A technology of adding, as well as Components a and b, an inorganic filler (Component c), polyethylene (Component d) and fatty acid amide or derivative thereof (Component e) at a predetermined ratio is also disclosed. Patent Document 1 however includes neither disclosure nor suggestion about the relationship among fluidity of the crystalline propylene polymer portion, a viscosity ratio of the crystalline propylene polymer portion to the ethylene•propylene random copolymer portion, and appearance of a molding typified by flow mark characteristic.
An object of Patent Document 2 is to provide a polypropylene resin composition which hardly generates flow marks or generates less black specks, and has excellent appearance when it is molded. As a means for resolution, a polypropylene resin (A) having a propylene homopolymer portion having an intrinsic viscosity [η]AP not greater than 1.3 dl/g and a propylene•ethylene random copolymer portion having an intrinsic viscosity [η]AEP not greater than 3.0 dl/g is disclosed. Moreover, a polypropylene resin composition obtained by mixing the resin A with ICP (resin B) different from the resin A in physical properties at a specific ratio is disclosed. In Patent Document 2, however, the number of flow marks is not reduced to the satisfactory level and mechanical physical properties or moldability is sacrificed to attain this.
An object of Patent Document 3 is to provide a polypropylene resin composition exhibiting good appearance and having excellent molding processability. As a means for resolution, use of a molding modifier made of ICP having specific physical properties is disclosed. Described specifically, ICP having, at a propylene homopolymer portion (crystal portion) thereof, an MFR of 500 g/10 min or greater, having, in its entirety, an MFR of 100 g/10 min or greater and having a die swell ratio of from 1.2 to 2.5 is disclosed. In Patent Document 3, however, occurrence of flow marks is not reduced to a satisfactory level. The mechanical physical properties (especially, low temperature impact strength) or moldability is sacrificed to reduce it.
[Patent Document 1] JP-A-2001-288331
[Patent Document 2] JP-A-2002-12734
[Patent Document 3] JP-A-2004-18647